1. Field of the Invention
The present invention generally relates to a test set for determining the operational status of an associated navigational satellite receiver, but more specifically, the present invention relates to a test set having the dual capability of isolating faults in a digital section of the associated navigational satellite receiver and generating a facsimile of a navigational satellite reference signal for isolating faults in a receiver processor section of the aforementioned associated navigational satellite receiver.
2. Description of the prior art
The AN/SRN-19(V)2 is an automatic shipboard radio navigational set that provides a continuous display of the position of the ship upon which it is associated. The ship's position, which is normally obtained by dead reckoning on the ship's true speed and heading, is periodically updated by fixes on a plurality of satellites. These satellites are part of the Navy Navigational Satellite System (NNSS), which is a world wide, all weather navigational system consisting of, inter alia, five satellites in polar orbits. The AN/SRN-19(V)2 shipboard radio navigational set, aforementioned, operates on a dedicated navigational program or algorithm which enables navigators to obtain accurate fixes using the data received from one of the five orbiting satellites.
Each satellite orbits the earth in approximately 107 minutes and continually transmits the following phase modulated data every two minutes on two radio frequency (RF) carriers: (1) time synchronization signals, (2) a 400 MHz signal, and (3) fixed and variable parameters describing the satellite's orbit. It should be mentioned that the foregoing satellite system is also used by commercial concerns.
The AN/SRN-19(V)2 radio navigational set comprises, inter alia, an antenna, an RF amplifier and a receiver-processor including a receiver portion and a data processor portion. The receiver portion extracts, amplifies and formats message information from the RF signal transmitted by the satellite and measures the doppler shift of this same signal. The reconstructed doppler shift of the satellite signal results from a relative motion between the receiver and the satellite. The message data obtained by the phase modulation of the RF carrier describes the satellite's position at an even two minute universal time period. The data processor processes inputs from the receiver, the ship's electromagnetic log and gyrocompass through two synchro-to-digital (S/D) converters and a receiver-processor keyboard. The data processor then performs computations and provides the desired outputs to a front panel display, a readout indicator, a printer and a cassette recorder.
It has been determined over a period of years that users of the NNSS were having problems determining whether the receiving equipment, for example, the AN/SRN-19(V)2, for obtaining their position was operating correctly, whether the receiving equipment was within the calibration range required for certification thereof, and, indeed, at times at sea (where references as to the ship's position were not available), whether the receiving equipment was operational.
Consequently, there is a need in the prior art for the capability of certifying satellite navigational receivers for operational readiness, and to facilitate and enable the end users to determine if there is, in fact, a failure and what the failed part is. Thus, the logistics could then be set-up so that instead of returning the particular navigational satellite receiver to a base depot for repair, the cognizant operator would be able to determine, in the field, what are the failed parts and request the particular replacement therefor.
The test set, according to the present invention, is configured to operate with the AN/SRN-19(V)2 radio navigational set, which is fully disclosed in the publication entitled, "Shipboard Operations and Maintenance Manual" (NAVELEX 0967-LP-634-9010).